The identification of antigenic structures encoded by the human immunodeficiency (HIV) genome is relevant for the development of vaccines and serological tests. Several highly reactive antigenic sites have been identified in the env and gag-encoded proteins of HIV-1 and HIV-2 (8-13). Peptides derived from the antigenic sequence located at the NH.sub.2 terminal of the transmembrane protein, allow a categorical distinction of antibodies to the two HIV types (12), whereas cross-reactivity between the two serotypes has been found for those regions showing sequence homology (12-14). However, serum reactivity to the gag and env cross-reacting regions is often very low and not all HIV-1 antibody positive sera score positive when tested in serological tests based on the cross-reacting peptides.
One of the features of the life cycle of retroviruses is the insertion of the proviral DNA into host chromosomes. A protein encoded by the 3' end of the pol gene of the virus genome has been shown to possess endonuclease activity (1) which is necessary for DNA integration. Sera from the majority of human immunodeficiency virus (HIV) infected individuals react with the endonuclease protein p31 in serological tests (2-4). It is not known, however, which part of the protein represents the target(s) for antibody response.